As industrial automation advances, interconnectivity between various microprocessor based plant floor devices, such as programmable logic controllers ("PLCs"), and plant computers, becomes more and more desirable. For example, the extensive math and register commands of a PLC can perform data pre-processing on raw data right at the raw data's point of origin, as opposed to uploading all of the raw data to the host computer, thereby permitting use of a smaller host computer.
Various schemes have been developed to interconnect PLCs and host computers, but their applications have been limited. For example, if one wanted to communicatively couple three PLCs in the absence of a network, each PLC would typically require a separate serial, or point to point, connection with each of the other two PLCs. However, the speed of serial communication is limited. Further, as the number of interconnected PLCs grows linearly, the number of serial connections grows geometrically.
In a co-pending, commonly assigned patent application Ser. No. 180,093, "now abandoned" a peer-to peer system is disclosed for interconnecting a plurality of PLCs. However this system requires a dedicated communication network.
Allen-Bradley Company, Inc., in conjunction with Digital Equipment Corporation ("DEC") has developed a system marketed under the trade name "Pyramid Integrator" for interconnecting devices over the relatively standardized Ethernet network via DEC's VAX.RTM. computer. However according to this system, only a maximum of four PLCs can be coupled to an Ethernet network per VAX computer, and each of the PLCs must be plugged into the backplane of the VAX computer. If five PLCs are required on the Ethernet, two VAX computers are required. This greatly adds to the expense of automation.
In addition, a host computer can concurrently perform a plurality of applications programs, or user tasks. When a PLC is connected to such a host computer, it is often important for the host computer to obtain data from the PLC. Typically this is accomplished by having the host computer poll the PLC. However, this polling either requires the host computer to interrupt the PLC's processing of its ladder program, or it requires the host computer to wait for the PLC to complete a scan of its ladder program. Further it is often important for the PLC to send unsolicited information to the host computer.
Messages typically are transmitted between microprocessor based devices on an Ethernet network in the form of data packets. The data packets generally include a preamble portion comprising routing information and protocol type, a user defined portion comprising the message itself, and an error detection portion. As the speed of communication between microprocessor based devices increases, error detection operations become ever more critical. Typically the error detection operation views the entire data packet to determine existence of an error. This often does not quickly enough detect errors in the user data portion. Further, the protocol often cannot accurately respond to lost messages.
Finally as automated systems control ever larger operations, handling and prioritizing of event notifications or alarms, such as faults, alerts and warnings, by the host computer becomes even more important. While certain host computers have been able to receive alarms, they have been received on a global basis, rather than individually on a user task basis.
Applicants' invention is provided to solve these and other problems.